The invention relates to multilayer combined rigid and flex printed circuits having flexible printed circuits extending from the rigid board.
The techniques of making multilayer rigid flex printed circuit boards is well known in the field. One early example of the prior art is disclosed in U.S. Pat. No. 3,409,732, assigned to the assignee of the present application. Typically a rigid flex stacked printed circuit board includes flexible printed circuit cables extending from the periphery of the rigid section or sections. The rigid portions of the flex cables are typically used as sites for electronic components or mechanical hardware. It is important to note that the copper conductor in each plane or layer is fabricated from one continuous sheet of copper foil.
With improvements in electronic technology, there has been a constant need for advances in electronic packaging. This need has led to more complex multilayer rigid flex printed circuit boards with many boards now using up to twenty-five, or even more, layers of circuitry. However, severe problems developed when the rigid circuit portions included many layers of conductors and holes plated through with copper to provide conductor barrels connecting the conductor layers.
Certain problems encountered are created by the thermal expansion of typically used insulator materials, such as acrylic adhesives and Kapton ("Kapton" is a trademark of E. I. du Pont de Nemours and Company Inc. for polyimide film) utilized in the construction of rigid flex boards. Thus failures occur when the board is subjected to elevated temperatures in thermal stress testing, hot oil solder reflow, and the like. The rate of thermal expansion (a fundamental material property) of the acrylic adhesive is about 30 percent, of Kapton about 10 percent, and of copper about 4 percent. When hot oil is used to reflow solder plated on the rigid printed circuit board, temperatures on the order of 450-500 degrees F. cause expansion, for example, of the acrylic adhesive used to bond Kapton layers to copper layers in the multilayer rigid sections. As temperatures increase, the board, which is unrestrained, grows much faster in the thickness, or Z direction, than "copper barrels" formed in the plated through holes in the multilayer rigid board section. The copper barrels stretch as the acrylic adhesive and Kapton expand, sometimes fracturing the copper. Repeated cycles tend to break many of the plated copper barrels found in the holes in the rigid board sections.
If less acrylic adhesive is used to limit expansion, the internal stresses developed during lamination procedures cause unacceptable voids or delaminations in the final board. Since these deficiencies are not apparent until the final stages of construction, costly scrapping of nearly completed boards is required.
It is now apparent that multilayer rigid flex boards including insulator materials such as acrylic adhesive and Kapton will always place Z-axis stress on plated through holes. The coefficient of thermal expansion of the acrylic adhesive (Z-axis expansion) is the dominant influence. Because of the amount of acrylic required in many multilayer rigid flex applications, all plated through holes will be stressed, with many of these cracking, making these boards unusable.
To overcome the foregoing problems, several fixes have been tried. These fixes include heavier copper plating in the holes, additional layers of material in the rigid flex board, and design variations such as pads versus no pads, in non-functional layers of the plated through holes. The problems, however, have not been overcome.
Another difficulty with the use of dielectric films such as Kapton in the rigid board area is their absorption of excessive moisture, on the order of up to 3 percent by weight of water. Absorbed moisture in the circuitry, which does not escape, may volatilize during high temperature operations and cause unacceptable delamination in the rigid board area. To remove the moisture from the Kapton and acrylic layers, the board must be baked at temperatures on the order of 250 degrees F. for many hours, for example 12, 24 or even 48 hours, an expensive process.
Yet another difficulty with insulator materials such as Kapton and acrylic adhesives involves the cleaning of holes, which have been drilled through the laminated rigid board, prior to plating through such holes. The excellent chemical resistance of the acrylic adhesive to typical cleaning solutions, precludes the use of cleaning solutions for removal of smears resulting from hole drilling. Also, acrylics are prone to swelling due either to exposure to cleaning fluids during smear removal or to improper smear removal processing. To avoid these problems, plasma etching is required to clean the holes, an expensive process.
Another, although lesser consideration, is the use of insulating layers such as glass layers in the flex cables. While considerable bending is possible with a glass layer in the laminated flexible areas, some applications require the cables to be flexed repeatedly through large angles resulting in fractures and other problems; thus, elimination of a glass layer from the flex cable is, in certain circumstances, desirable.